1. Field of the Invention
This invention relates to imaging radar systems. Particularly, this invention relates to high frequency real aperture three-dimensional radar imaging systems.
2. Description of the Related Art
Numerous commercial technologies can detect weapons or contraband concealed in clothing on persons—from trace chemical sniffers to X-ray imagers—but in almost all cases these approaches require the sensor and the target to be in close proximity. For situations that call for remote detection, such as when hidden explosives may be detonated or where clandestine surveillance is warranted, concealed weapons detection is at best extremely difficult to accomplish.
Conventional radars are being widely investigated for this purpose, but stringent spatial resolution requirements make these systems impractical because of the large bandwidths and aperture sizes needed. Traditional radar systems are also poorly suited for spectroscopic identification of materials such as explosives. Some progress in through-clothing imaging has been reported using passive thermal detectors in the submillimeter spectrum, but these approaches are lacking in sensitivity and spectral selectivity.
Among the conventional detection systems employing radar techniques for human targets, many operate only by detecting power of a beam reflected off a target. Such radar detection systems infer a characteristic of the object reflecting power from a location compared against reflected power from other locations in a two-dimensional scan. Radar detection systems operating in this manner typically do not derive or utilize range information across the target object (the person). Thus, such radar systems operating based on reflected power alone are not three-dimensional imaging systems. In contrast, radar imaging systems employ derived range information to a target, typically ignoring reflected power. However, effective three-dimensional radar imaging systems can be difficult to produce. Imaging technology in the THz range has primarily focused on acquiring two-dimensional camera-like representations of a scene. However, additional utility would come from a fully three-dimensional imaging radar with high range resolution.
Recent progress in terahertz (THz) technology, as well as the demand for new surveillance capabilities, has led to the development of prototype submillimeter imagers capable of detecting weapons concealed within clothing or packages. See, e.g. McMillan, “Terahertz Imaging Millimeter-Wave Radar,” Advances in Sensing with Security Applications Digest, NATO Advanced Study Institute, II Ciocco Italy, pp. 1-26, Jul. 17-30, 2005 (http://w.nato-asi.org/sensors2005/papers/mcmillan.pdf); Dengler, “Passive and Active Imaging of Humans for Contraband Detection at 610 GHz,” 2004 IEEE MTT-S Intl. Microwave Sym. Digest, Ft. Worth, Tex., June 2004, pp. 1591-1594; Petkie et al., “Active and passive millimeter and sub-millimeter-wave imaging” Proc. SPIE, vol. 5989, pp. 598918-1 to 598918-8, 2005; Dickinson et al., “Terahertz imaging of subjects with concealed weapons,” Proc. SPIE, vol. 6212, pp. 62120Q-1 to 62120Q-12, 2006; Kemp et al., “Security applications of terahertz technology,” Proc. SPIE, vol. 5070, pp. 44-52.2003; and Dengler et al., “A Compact 600 GHz Electronically Tunable Vector Measurement System for Submillimeter Wave Imaging,” 2006, IEEE MTT-S Intl. Microwave Symp. Digest, San Francisco, Calif., June 2006, pp. 1923-1926, which are all incorporated by reference herein. Imaging in the THz regime is attractive because wavelengths in the range 100 μm<λ<0.5 mm are short enough to provide high resolution with modest apertures and long enough to penetrate materials such as cloth or cardboard.
With the ability to penetrate clothing, the potential for cm-scale image resolution, and SNR in excess of 106, it is no wonder that active (illuminated) submillimeter-wavelength imaging has attracted great interest for standoff weapons detection. However, substantial sensitivity and good resolution are insufficient to reliably detect concealed objects. Current approaches to THz imaging do not yet meet all of the real-world and often conflicting requirements of standoff range, portability, high speed, penetrability, target identification, and cost.
U.S. Pat. No. 7,345,279 by Mueller, issued Mar. 18, 2008 discloses a method for inspecting a package to identify an object concealed in the package includes passing two beams of THz-radiation through the package. The frequency of THz radiation in one beam is different from that in the other, and the beams are at an angle to each other. Each of the transmitted beams is used to form an image of the package and the object. The absorption coefficient of the object is determined from the two images. The material of the object is determined from the absorption coefficients at the two frequencies. The method is useful for detecting explosive material concealed in baggage.
U.S. Patent Publication No. 20060214107 by Mueller, published Sep. 28, 2008 discloses a THz-frequency heterodyne imaging method is used to remotely detect objects concealed in or under a person's clothing. One THz-frequency beam is scanned over a person being examined. A portion of the beam penetrates the persons clothing and is reflected by an object concealed under the person's clothing. The reflected portion the beam is mixed with another beam of THz-frequency radiation having a different frequency to provide a signal having an intermediate frequency (IF) including image data representative of the concealed object.
Mueller employs THz radiation is generated through optical pumping of a CO2 laser, and a heterodyne detection technique is used to measure the reflected signal. Because of the ability for THz to penetrate clothing, Mueller speculates that hidden contraband can be detected based on the downconverted signal. Mueller also speculates that high-resolution radar techniques would assist in detecting hidden objects, and a frequency-modulated continuous-wave (FMCW) radar technique is proposed to accomplish that. However, no algorithmic description explaining how radar data is to be used for object detection is given, and no methods of FM-chirp nonlinearity compensation are described.
In real-life scenarios a coherent radar image will typically exhibit very poor contrast between a concealed object and the surrounding clothing and skin—even for hidden metallic objects such as guns. The challenge of actively illuminated submillimeter wave detection of concealed objects involves extracting signals from scene clutter rather than from noise. For example, while active THz imaging systems using high power coherent illumination and ultra-low-noise heterodyne detection show great promise, they often face operational drawbacks such as requiring cryogenic detectors or bulky laser sources. A more fundamental difficulty with coherent active imaging is that by relying on a single frequency, target recognition is reliant on an object's contrast and brightness which, in turn, are highly sensitive to incidence angle of radiation, clutter signal from the foreground or background, and interference and speckle effects.
In view of the foregoing, there is a need in the art for apparatuses and methods for high frequency radar providing three-dimensional imaging with high range resolution. There is also a need for such apparatuses and methods employing long standoff range, speed and penetrability. There is a need for such apparatuses and methods to operate with reduced sensitivity to incidence angle of radiation, clutter signal from the foreground or background, and interference and speckle effects, indicative of other imagers. There is further a need for such apparatuses and methods to operate allow conceal target identification at reasonable cost. There is particularly a need for such apparatuses and methods in security applications to detect concealed weapons and explosives on individuals. These and other needs are met by the present invention as detailed hereafter.